Rotating equipment will usually have unbalance in the as fabricated state. Operation with such unbalance creates vibration and damage. It is accordingly well known to balance the as fabricated rotor prior to use.
In balancing the typical turbine rotor, it is rotated at some speed such as 900 RPM with balancing machines capable of detecting the angle and amount of the unbalance. If an infinite gradation of counterweights were available and an infinite number of locations were available, it would be a simple matter to place the appropriate weight precisely opposite the measured unbalance location.
Typically, however, because of inventory problems only certain weights are available and because of design requirements only certain preselected locations are available which may accept the counterweights. Accordingly, the counterweights are added at various locations in an attempt to correct the initial unbalance of the rotor and the rotor is again spun and the unbalance measured. A design specification is conventionally set as the maximum acceptable unbalance. If the residual unbalance is not below the specification, they are changed and a new residual unbalance is measured. This iterative trial and error process continues for some time until a combination resulting in an unbalance below the limit is found.
Even when a combination of weights is found which corrects the unbalance to below specification, there is no way of knowing whether or not lowest possible residual unbalance has been attained. While such a selection may meet the specifications, it still results in vibration and long term damage which is in excess of that which would occur had the optimum selection been used.
It is also desirable to use a limited number of weights to minimize stresses and total weight of the engine.
It is an object of the invention to obtain the minimum possible residual unbalance operating within the constraints of weight sizes, weight locations and amount of weights.